are required to have speed monitoring devices fitted. Signage Historical limits Historically, Australia operated a simple speed limit system of urban and rural default limits, denoted in miles per hour. As part of metrication in 1974, speed limits and speed advisories were converted into kilometres per hour, rounded to the nearest 10 km/h, leading to small discrepancies in speed limits. Also, the signage was changed from the design where the words "SPEED LIMIT" appear above the numeric limit (as specified in the current US MUTCD) to the design where the numeric limit is inscribed on a red circle (which is defined by the Vienna Convention and thus it became an international sign for speed limit). Urban limits The urban default, which prior to the 1930s was , applied to any "built up area", usually defined by the presence of street lighting. Over the next 30 years, each of the states and territories progressively increased the limit to , with New South Wales being the last to change in May 1964. South Australia adopted on 30 November 1950, along with the "new short-right hand turn" in place of a hook turn. Metrication led to the default urban limits of being converted to 60 km/h, an increase of . In the late 1990s and early 2000s, the urban default limit was progressively lowered to nationally for reasons of road, and especially pedestrian, safety. However, many existing roads, especially subarterial roads in urban areas, have had limits posted on them. Queensland's Manual of Uniform Traffic Control Devices (Speed Controls) states that is the general minimum speed limit for traffic-carrying roads. The Northern Territory has retained the limit; however, is also a common speed limit (particularly in residential areas). Rural limits Outside of built up areas, a prima facie speed limit applied. In New South Wales and Victoria, speed limit was 50 miles per hour (80 km/h in New South Wales after metrication). In the 1970s however, most state speed limits were gradually replaced by absolute limits. An absolute speed limit of was introduced to Victoria in 1971, as a trial. This was subsequently reduced to in late 1973. South Australia introduced an absolute speed limit of in 1974. With metrication in 1974, the rural defaults of and became and respectively. The 50 mph limit in New South Wales became a limit of 80 km/h. New South Wales introduced an absolute speed limit of